The objective of the study was to determine which psychoactive prescription drugs are illegally obtained and through which ways of acquisition. OPPIDUM is an annual national study. It is based on specialized care centers that included subjects presenting a drug dependency or under opiate maintenance treatment. All their psychoactive substances consumed are reported. This work focuses on the different ways of acquisition specially the illegal ways of acquisition (bought on the street, forged prescription, stolen, given, internet). For each medication illegally obtained, a ratio has been calculated (number of illegal acquisitions divided by the number of described acquisitions). In 2008, 5542 subjects have been included and have described the consumption of 11 027 substances including 63.8% of prescription drugs. Among them, 11% were illegally obtained. The different illegal acquisition ways were 'street market' (77.6%), 'gift' (16.6%), 'theft' (2.3%), 'forged prescription' (2.3%), and 'internet' (0.7%). The third first drugs illegally obtained were high dosage buprenorphine, methadone, and clonazepam. Some prescription drugs, less consumed, have an important ratio of illegal acquisition like ketamine, flunitrazepam, morphine, trihexyphenidyl, or methylphenidate. This study confirms that theft, forged prescription and internet are few used and permits to highlight diversion of prescription drugs. It is important to inform healthcare professionals on the different prescription drugs that are illegally obtained.